1. Field of the Invention
The invention relates to a package for a strain sensor.
2. Description of the Related Art
Many applications call for strain measurements relating to static and dynamic loading of structures and components and for the subsequent derivation of information from such measurements. From U.S. Pat. No. 5,585,571 a method and apparatus for measuring strain has been made known. The method and apparatus for measuring strain as described in U.S. Pat. No. 5,585,571 is based on a so-called surface acoustic wave resonator (SAW-resonator). Such a resonator is made up of a microstructure deposited on a piezoelectric substrate. The microstructure is formed by at least one pair of interleaved comb-like (“interdigitated”) electrodes deposited as a thin metal conducting layer on a substrate. In FIGS. 5 and 6 a basic model of a one port surface acoustic wave resonator used as a strain sensor is shown.
The strain sensor on basis of a surface acoustic wave (SAW) resonator is described now in further detail. The resonator has at least one interdigital transducer (IDT) with two electrical terminals and it may also contain two reflecting periodic gratings positioned one on each side of the transducer. The RF signal from the signal source applied to the IDT terminal excites surface acoustic waves propagating in opposite directions. As a result of reflections of SAW from the gratings a resonance with a very high quality (Q) factor (of the order of 10,000) occurs at the frequency of several hundred megahertz. The resonant frequency of the SAW resonator can be measured in a contactless way by an electronic interrogation unit. In the event that the substrate of the SAW resonator is subject to strain, the periods of the gratings and the IDT change and the SAW propagation velocity also changes proportionally to the strain value. As a result, the resonant frequency also changes proportionally to the strain. By measuring the change in the SAW resonant frequency one can measure the strain. The SAW strain sensing element may contain several electrically connected SAW resonators in order to exclude or compensate for the influence of temperature, the variation of electric parameters of the RF communication channel and some undesirable mechanical forces. In particular, when measuring torque applied to a shaft by way of measuring the direct strain components (tension and compression) of the shear strain on its surface, there may be two SAW resonators positioned on a single substrate at 45° to the shaft axis. In this case the difference between the two resonant frequencies is proportional to the torque value.
A third SAW resonator may also be positioned on the same single substrate in order to sense temperature, thereby permitting temperature compensated torque to be determined from a single SAW sensor.
The operational frequencies of an SAW resonator can be selected in a wide frequency range extending from a few megahertz up to a few gigahertz. The SAW resonator can be operated in many different modes as described for example in U.S. Pat. No. 5,585,571 whose content is fully incorporated in this application by reference. The choice of the operating mode depends on the strain measurement which should be undertaken.
In order to prevent the SAW substrate from environmental harm, the SAW substrate should be enclosed within a hermetic package. According to the state of the art, SAW substrates are packaged in hermetic packages consisting of two pieces made from materials such as austenitic stainless steel. The two pieces included a flat base part and a top head part, which are assembled together, for example by welding. The packages furthermore also have an electrical leadthrough, which is made hermetic by use of a glass fritted seal. The glass fritted seal functions either by a difference in the expansion coefficient, known as a compression seal, or by bonding to a preoxidized surface layer known as a matched seal. The compression seal is normally used in austenitic stainless steel packages, whereas the matched seal is used in a nickel-cobalt ferrous alloy such as KOVAR® packages.
In the state of the art, the SAW substrate used in a strain sensor is die bonded to the base part of the package. The base part is then bonded or welded to the component from which the strain should be determined. For determining the strain, it is necessary, that the strain is transferred from the component by way of the base part onto which the SAW substrate is bonded to the SAW resonator or resonators elastically, i.e. without hysteresis, plastic or creep deformation.
The two piece packages suffer from the problems that either they have poor elastic properties or that the glass fritted compression seal, throughwhich the leadthroughs are guided, cracks.
From US Patent Application Publication No. 2006/0130585 A1 a package for a torque SAW resonator has been made known. The package system made known from US 2006/0130585 A1 is designed to maximize the strain in the torque SAW resonator. The base part of the package is made from a high strength stainless steel, whereas the package wall is formed from a more flexible stainless steel. A disadvantage of this package is that the term “flexible” is not defined. All steels, whether martensitic, austenitic or precipitation hardening have a similar Young's (E) Modulus of around 200 GPa which defines the slope of the elastic stress-strain line and therefore the intrinsic elastic stiffness of the material. In addition US 2006/0130585 describes neither the stress-strain state of the materials indicated nor the manufacturing process, both of which are critical.
From U.S. Pat. Nos. 4,213,104 and 4,422,055 surface acoustic wave devices have been made known. The package of the devices shown in U.S. Pat. Nos. 4,213,104 and 4,422,055 include a base, which is the SAW substrate material, a spacer and a cover. In U.S. Pat. No. 4,213,104, all parts are made are made from a material having substantially the same thermal expansion characteristics and crystallographic orientation. Since the requirements for a SAW substrate are a piezo-electric crystalline material, such as quartz, this package is characteristic of an “all quartz” package.
In U.S. Pat. No. 4,422,055, a cut (circular slot) is introduced which surrounds the SAW couplers in order to isolate them from external strains.
In GB 2,346,493 a package for a strain sensor is disclosed. As in U.S. Pat. Nos. 4,213,104 and 4,422,055 the base (SAW substrate material) is quartz as is the lid. The central part is a thin window of adhesive film, so that this is another design approach to an “all quartz” package.
“All quartz” packages have potential cost advantages, however their intrinsic brittle nature (no capacity for inelastic deformation) means they are less tolerant of mechanical shock and cannot be attached to a structural component by means such as welding which are desirable for high volume automotive applications.
It is therefore an object of the invention to provide a package for a strain sensor which avoids the disadvantages mentioned in the state of the art. Especially a package for a strain sensor should be provided which can be mounted to a structural component so that good mechanical coupling is provided enabling elastic strain transfer to the SAW substrate which is bonded inside the package.